Next to the "To" field is the fx button that opens the formula editor. Click that.
In the formula editor, use Insert Function. You are looking for concat.
The concat formula will look like:
concat(yourIdField, "@domain.com")
So, the first bit, that says "yourIdField" you use the Insert Field or Group and go get your ID field from the main data source. The second bit, inside the quote marks, is the additional string that will make up the email address when the formula is concatenated.
Please attach your form to the Options tab if this is not making sense for you, and I can add the formula for you so that you can see it.